Production of melamine



Patented Apr. 17,1951

UNITED STATES PATENT OFFICE PRODUCTION OF MELAMINE Donald D. Lee, Wilmington, Del., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application December 28, 1944, Serial No. 570,215

This invention relates to the manufacture of melamine and particularly to methods for removing melamine from reaction vessels in which it is produced. The invention is especially useful in separating melamine from products of high pressure urea pyrolysis, such as are obtained in accordance with the process described in the copending application of D. J. Loder, S. N. 504,164, filed September 28, 1943.

For many years, one of the troublesome problems in the manufacture of melamine hasbeen the removal of hot melamine from vessels in which it was synthesized. It was heretofore con- 11 Claims. (01. 260-2497) sidered that none of the common solvents was suitable for this purpose, since it was believed the molten state, but this involved several engineering problems, such as the use of high temperature let-down valves, which in general have been known to be a frequent source of troublesome operation. Moreover, in the method for synthesis of melamine by pyrolysis of urea, in accordance with the aforementioned copending applications, additional problems have been confronted. Thus, under certain conditions, the pyrolysis of urea gives a reaction product which may contain substantial amounts of unconverted urea. Therefore, it has been desirable to devise a suitable simple means for completely removing such urea from the melamine associated therewith.

An object of this invention is to provide a satisfactory method for withdrawing melamine or mixture containing melamine from a reaction vessel in which it is produced. A further object is to separate melamine from the products obtained'in the continuouspyrolysis of urea. A still further object is to provide a simple inexpensive method for separating melamine from mixtures containing melamine and urea.

These and other objects are accomplished in accordance with the invention by subjecting the melamine-containing reaction mixture obtained by pyrolysis of urea to the action of water at elevated temperatures and pressures. In a specific embodiment water is pumped into the exit portion of a high-pressure urea pyrolysis unit, where it comes in contact with a molten urea'pyrolyzate, the chief ingredient of which is melamine. at a temperature within the range of .275" to 550 0., whereby the reaction mixture is quenched to a temperature of about 70 to 225- 0. During this quenching process any urea pres- 2 ent is almost instantly destroyed, but the melamine remains substantially unchanged. Thus, it has been discovered in accordance with this invention that when the urea pyrolyzate having a temperature of 275 to 550 C. is quenched with water to a temperature of 7 0 to 225 C. under superatmospheric pressures (generally at pressures of about 100 to 1000 atmospheres and. preferably from 200 to 600 atmospheres) the melamine dissolves in Water without substantial decomposition. This aqueous melamine solution can be pumped, valved, filtered, or otherwise handled with considerable ease.

The amount of water which may be employed in the practice of the invention is in most instances from about 2 to 50 times, preferably about 8 to 20 times (by weight) the weight of melamine to be withdrawn. The amount of water used generally depends primarily upon the desired temperature of the aqueous efiluent. Other considerations such as the amount of free ammonia present also have a bearing on the optimum amount of water employed. In general, the amount of water is controlled so as to give an efiiuent having a temperature within the preferred range of about 140 to 175 0., since melamine is quite soluble in water under these conditions (solubility of melamine in water at 150,

- 21 grams per 100 grams of water).

the exit portion of the converter.

The water with which the pyrolyzate is quenched may contain various dissolved materials if desired. As an illustration it is frequently convenient to employ, as an aqueous quench, the mother liquor obtained in melamine crystallization processes. If desired, the quench may be preheated, suitably to a temperature of about 30 to 200 C. v

This invention is further illustrated by means of the following examples: 1 r

Example 'l-Molten urea is conducted through a silver-lined tubular converter (length 40 inches) having an inside diameter of 1% inch under a pressure of 400 atmospheres at 400 C. The resulting fluid mixture containing melamine and urea is subjected to the action of a stream of water (about 4 parts by weight per part of urea charged; equivalent to about 16 parts per part of melamine present) said water being injected into The resulting aqueous solution, which is still under a pressure of 400 atmospheres, has a temperature of C. This aqueous solution can be readily pumped, valved, and filtered. A filtrate obtained from this solution, upon cooling, yields crystalline mela mine. The mother liquors are found to be free of urea, which indicates that the unconverted urea had been completely consumed by the action of the injected water.

Example 2.A mixture of urea and ammonia containing 4.2 moles of ammonia per mole of urea is passed through a silver-lined tube at a temperature of 400 C. under a pressure of 400 atmospheres, contact time being about minutes. At the exit end of the converter the hot reaction mixture consisting of ammonia, carbon dioxide, and a molten urea pyrolyzate is quenched by injection of water (about 20 parts by weight per part of urea charged) which causes the formation of an aqueous solution of melamine. The pressure on this solution is let down by means of a heated valve to about to 50 pounds per square inch, which causes the liberation of ammonia and carbon dioxide. Finally the pressure is reduced to atmospheric pressure. mixture is then evaporated at 100 C. until crysstals appear, then cooled to room temperature. The resulting crystals are removed by filtration and are subsequently dried. This product is found to be substantially pure melamine.

Example 2.An aqueous mother liquor similar to that obtained in the crystallization of melamine, in accordance with the preceding example, is preheated under a pressure of 400 atmospheres to a temperature of 150 C. It is then injected into the exit portion of a continuous urea pyrolysis unit operating at a temperature of 400 C. under a pressure of 400 atmospheres. The resulting aqueous solution of melamine is passed through a letdown valve, and is filtered while hot to remove small amounts of insoluble impurities. Upon cooling the filtrate, crystalline melamine precipitates. The crystals are removed by filtration and are air-dried, yielding pure melamine.

It will be understood that the above examples are illustrative only and that the invention is not limited thereto. Numerous modifications of the invention will occur to those who are skilled in the art. Any suitable pressure resistant apparatus may be employed in the practice of the invention. For example, various heretofore known devices may be employed for adequately contacting the injected water with the molten melamine-containing mixture. However, in general, a simple T tube accomplishes this result in a satisfactory manner.

Since many diiferent embodiments of the invention may be made without departing from the spirit and scope thereof, it will be understood that we do not limit ourselves except as set forth in the following claims.

I claim: 1.-A process for removing melamine from a reaction vessel which comprises quenching a melamine-containing urea pyrolyzate having a temperature of 275 to 550 C. by direct contact with from 2 to 50 parts of Water per part of melamine at a temperature above 140 C. under super-atmospheric pressure whereby an aqueous solution of melamine is produced, withdrawing the said aqueous solution from the said reaction vessel, and cooling the said aqueous solution whereby crystallization of melamine occurs.

2. A process for removing melamine from a reaction vessel which comprises quenching a melamine-containing urea pyrolyzate having a temperature of 275 to 550 C. by direct contact with from 2 to 50 parts of Water per part of melamine at a pressure within the range of 200 $9109 a mo phe es wh by an q e l The 4 tion of melamine having a temperature of about 140 to 175 C. is produced, withdrawing the said aqueous solution from the said reaction vessel, and cooling the said aqueous solution, whereby crystallization of melamine occurs.

3. A process for removing melamine from a reaction vessel which comprises quenching a melamine-containing urea pyrolyzate having a temperature of 275 to 550 C. by direct contact with from 8 to 20 parts of water per part of melamine at a pressure within the range of 200 to 600 atmospheres whereby an aqueous solution of melamine having a temperature of about 140 to 175 C. is produced, withdrawing the said aqueous solution from the said reaction vessel, and cooling the said aqueous solution whereby crystallization of melamine occurs.

4. A process for removing melamine from a reaction vessel containing a urea pyrolyzate consisting essentially of urea and melamine, which comprises quenching the said pyrolyzate having a temperature of- 275 to 550 C. by direct contact with from 2 to 50 parts of water per part of melamine at a pressure within the range of to 1000 atmospheres whereby a urea-free aqueous solution of melamine having a temperature of about to C. is produced, withdrawing the said aqueous solution from the said reaction vessel, and cooling the said aqueous solution whereby crystallization of melamine occurs.

5. A process for removing melamine from a reaction vessel containing a urea pyrolyzate consisting essentially of urea and melamine, which comprises quenching the said pyrolyzate having a temperature of 275 to 550 C. by direct contact with from 8 to 20 parts of water per part of melamine at a pressure within the range of 200 to 600 atmospheres, whereby a urea-free aqueous solution of melamine having a temperature of about 140 to 175 C. is produced, withdrawing the said aqueous solution from the said reaction vessel, and cooling the said aqueous solution whereby crystallization of melamine occurs.

6. A process for removing melamine from a reaction vessel containing a molten urea pyrolyzate, the chief ingredient of which is melamine, which comprises quenching the said pyrolyzate having a temperature of 275 to 550 C. by direct contact with from 2 to 50 parts of water per part of melamine at a pressure within the range of 100 to 1000 atmospheres, whereby an aqueous solution of melamine having a temperature of about 140 to 175 C. is produced, thereafter withdrawing the said aqueous solution from the said reaction vessel, and cooling the said aqueous solution whereby crystallization of melamine occurs 7. A process for removing melamine from a reaction vessel containing a molten urea pyrolyzate, the chief ingredient of which is melamine, which comprises quenching the said pyrolyzate having a temperature of 275 to 550 C. by direct contact with from 8 to 20 parts of water per part of melamine at a pressure within the range of 200 to 600 atmospheres, whereby an aqueous solution of melamine having a temperature of about 140 to 175 'C."lS produced, there-,- after withdrawing the said aqueous solution from the said reaction vessel, and cooling the said aqueous solution whereby crystallization of melamine occurs.

8. A-process for removing melamine from a reaction vessel containing a molten urea pyrolyzate, the chief ingredient of which is melamine, w ch compr ses. ,quenQhinsthe ai pyrolyzate contact with water under a pressure of about 400 atmospheres, whereby an aqueous solution of melamine having a temperature of about 140 to 175 C. is produced, thereafter withdrawing the said aqueous solution from the said reaction vessel, and cooling the said aqueous solution whereby crystallization of melamine occurs.

9. A process for removing melamine from a reaction vessel containing ammonia, carbon dioxide and a molten urea pyrolyzate, the chief ingredient of which is melamine, which comprises quenching the said ammonia, carbon dioxide and molten urea pyrolyzate having a temperature of about 400 C. by direct contact with water at a temperature above 140 C. under a pressure of about 400 atmospheres, and thereafter lowering the pressure on the resulting aqueous solution to about 15 to 50 pounds per square inch, whereby dissolved ammonia and carbon dioxide are liberated, and cooling the resu ting equeous solution, whereby separation of melamine crystals occurs.

10. In a process for isolating melamine, the steps which comprise heating a melamine-containing urea pyrolyzate with from 2 to 50 parts by weight of water per part of melamine whereby an aqueous solution of melamine having a temperature of 140 to 225 C. is produced, and thereafter cooling the resulting mixture, whereby crystallization of melamine occurs.

11. In a process for isolating melamine, the steps which comprise heating a melamine-containing urea pyrolyzate with from 2 to parts by weight of Water per part of melamine whereby an aqueous solution of melamine having a temperature of to C. is produced, and thereafter cooling the resulting mixture, Whereby crystallization of melamine occurs.

DONALD D. LEE.

REFERENCES CITED he following references are of record in the of this patent:

UNITED STATES PATENTS OTHER REFERENCES Davis, J. Amer. Chem. Soc. vol. 43 (1921), pp. 2236-2233.

Industrial and Engineering Chemistry, vol. 32, No. 9, pp. 1183, 1184. 

1. A PROCESS FOR REMOVING MELAMINE FROM A REACTION VESSEL WHICH COMPRISES QUENCHING A MELAMINE-CONTAINING UREA PYROLYZATE HAVING A TEMPERATURE OF 275* TO 550* C. BY DIRECT CONTACT WITH FROM 2 TO 50 PARTS OF WATER PER PART OF MELAMINE AT A TEMPERATURE ABOVE 140* C. UNDER SUPER-ATMOSPHERIC PRESSURE WHEREBY AN AQUEOUS SOLUTION OF MELAMINE IS PRODUCED, WITHDRAWING THE SAID AQUEOUS SOLUTION FROM THE SAID REACTION VESSEL, AND COOLING THE SAID AQUEOUS SOLUTION WHEREBY CRYSTALLIZATION OF MELAMINE OCCURS. 